1. Field of the Invention
The present invention relates to wireless communications and, more particularly, to a method and system for controlling the power level of signals transmitted over a cellular air interface between a base station and a mobile station.
2. Description of Related Art
Cellular wireless is an increasingly popular means of personal communication in the modern world. People are using cellular wireless networks for the exchange of voice and data over cellular telephones, Personal Digital Assistants (“PDAs”), cellular telephone modems, and other devices. In principle, a user can seek information over the Internet or call anyone over a Public Switched Telephone Network (“PSTN”) from any place inside the coverage area of the cellular wireless network.
In a typical cellular wireless system, an area is divided geographically into a number of cell sites, each defined by a radio frequency (“RF”) radiation pattern from a respective base transceiver station (“BTS”) antenna. The base station antennae in the cells are in turn coupled to a base station controller (“BSC”), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (“MSC”) for instance. The MSC may then be coupled to a telecommunications network such as the PSTN (public switched telephone network) or the Internet.
When a mobile station (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) (“MS”) is positioned in a cell, the MS and BTS can communicate with each other in various channels over the RF air interface. Communications from the BTS to an MS are considered to be in a “forward” direction, so the air interface channels used to carry such communications are referred to as the “forward link” channels. Conversely, communications from an MS to the BTS are considered to be in a “reverse” direction, so the air interface channels used to carry such communications are referred to as “reverse link” channels.
MS-BTS communications will typically suffer from varying levels of interference and signal degradation, due to factors such as (i) the number and power level of mobile stations concurrently communicating over the air interface, (ii) obstructions such as buildings or foliage, and (iii) the distance between the MS and the BTS. In order to account for this, the power level of signals transmitted between the MS and BTS can be dynamically adjusted.
For instance, in a Code Division Multiple Access (“CDMA”) system operating according to industry standard IS-2000-2 (published in March 2000 as TIA/EIAIS-2000.2-A), the BSC and MS will engage in a two-part power control process for reverse link transmissions. In this process, the BSC will maintain a “setpoint,” Eb/No, which is a decibel measure of the MS signal energy to noise (spectral density). The setpoint represents how strong the MS signal must be for the BTS to be able to successfully receive bits of data transmitted by the MS. Given a particular noise level in the air interface, if the received MS signal level is not high enough, the BTS might not be able to make out the bits of the signal compared to the background noise.
A typical BTS will have an initial setpoint designated by the manufacturer of the BTS for use with respect to all mobile stations that the BTS serves. Further, a BSC will continuously estimate the noise level in the air interface, based on various factors such as frame error rate or signal strength measurements reported by mobile stations, for instance.
When an MS is going to engage in a call, it will select an initial reverse link power level (i.e., the power level at which it will initially transmit to the BTS). Typically, the MS selects the initial transmit power based on its measurement of received signal power levels. For instance, if the MS receives a relatively high strength signal, it might logically conclude that it is relatively close to the base station, so it might be programmed to transmit to the BTS at a relatively low power level. Conversely, if the MS receives a relatively low level signal, it might be programmed to transmit to the BTS at a relatively high power level.
The first part of the reverse link power control process is called “open loop power control.” In open loop power control, the BTS will measure the power level of the signal that it receives from the MS, which will have degraded from the time that it left the MS, and the BTS will provide the BSC with the measurement. Given this value and the BSC's estimate of noise in the air interface, the BSC will then establish a measured value of Eb/No, which the BSC will compare to the setpoint. If the measured Eb/No does not match the setpoint, the BSC will instruct the MS to adjust its transmit power, typically by a predetermined increment. The BSC will then establish a new measured Eb/No value and compare it to the setpoint, repeating the process until the measured Eb/No matches the setpoint.
For instance, if the measured Eb/No is too low compared to the setpoint, then the BSC will conclude that the MS needs to increase its transmit power and will therefore send a signaling message (a bit in a predetermined slot) to the MS instructing the MS to increase its transmit power, typically by 1 dB or another increment specified by the BSC. Similarly, if the measured Eb/No is too high compared to the setpoint, then the BSC will conclude that the MS needs to decrease its transmit power and will therefore send a signaling message to the MS instructing the MS to decrease its transmit power, also typically by a 1 dB increment.
As an example, assume that (i) the initial setpoint is 5 dB, (ii) the MS transmits at 10 dBm and (iii) the estimated noise level is −65 dBm. Assume then that the BTS measures received MS signal energy of −61 dBm. The BSC will therefore compute a measured Eb/No of 4 dB, which is lower than the setpoint by 1 dB. Consequently, the BSC will instruct the MS to raise its transmit power by 1 dB. As a result, the BTS might measure received MS signal energy of −60 dBm. The BSC will then compute a measured Eb/No of 5 dB, which matches the setpoint. After matching the setpoint, communication has been successfully established between the BTS and the MS. The BTS and the MS will then enter into the next part of the power control process, referred to as “closed loop power control.”
In closed loop power control, the BSC will continuously measure the frame error rate (FER) of received MS signals, i.e., the percentage of frames on the reverse link that are in error, and the BSC will compare it to a predetermined FER threshold. Further, the BSC will continue to measure the Eb/No and compare it to the setpoint. Typically, the BSC will update and compare the FER at every 20 millisecond frame, and the BSC will update and compare the Eb/No every 1.25 milliseconds.
If the measured FER is greater than a predetermined threshold, the BSC will conclude that the setpoint should be increased. Therefore, the BSC will increase the setpoint by an increment, such as 1 dB for instance. As a result, however, the setpoint may be pushed higher than the measured Eb/No. Therefore, the BSC will instruct the MS to increase its transmit power.
On the other hand, if the measured FER is less than the predetermined threshold, the BSC will conclude that the setpoint should be reduced. Therefore, the BSC will reduce the setpoint, similarly by an increment such as 1 dB for instance. As a result, the setpoint may be pushed lower than the measured Eb/No. Therefore, the BSC will instruct the MS to reduce its transmit power.
Note that, in addition to the initial setpoint, a BTS will also have maximum and minimum settings for the reverse link setpoint. These maximum and minimum settings will define bounds beyond which the BSC will not adjust the setpoint during closed loop power control.
To illustrate this closed loop power control process, continue further with the example above. In the example, assume, for instance, that (i) the FER threshold is 2%, (ii) the BSC measures an FER of 3% and a received MS signal energy of −60 dBm, and (iii) the estimated noise level remains at −65 dBm. Because the measured FER exceeds the threshold FER, the BSC will increase the setpoint by 1 dB, to establish a new setpoint of 6 dB. However, the BSC will compute a measured Eb/No of 5 dB, which is lower than the new setpoint. Consequently, the BSC will instruct the MS to increase its transmit power by 1 dB, which may cause the received MS signal energy to be −59 dBm. As a result, the measured Eb/No will be 6 dB, which will match the new setpoint.
A similar power control process occurs for transmissions on the forward link. On the forward link, according to IS-2000, a BTS may have up to 61 traffic channels, for communicating with up to 61 different mobile stations at once (on a given frequency). Each traffic channel is defined by a separate “Walsh code,” which is used as a basis for the BSC to encode and the MS to decode communications on the channel. For a given channel, the BTS will transmit to the MS at a certain power level, which is typically defined in party by a Digital Gain Unit (“DGU”) parameter stored by the BSC. Specifically, a BTS manufacturer may specify a particular scale or algorithm to translate a DGU parameter into a forward link traffic channel power level.
In operation for a given traffic channel, the BSC will select an initial DGU and will translate the initial DGU into an initial forward link power level, at which the BTS will transmit traffic to the MS. As the MS receives the traffic, the MS will monitor the FER and will report the FER to the BTS in an IS-2000 Power Measurement Report Message (“PMRM”). When the BTS receives the PMRM, the BSC will determine if the FER is greater than a predetermined threshold (such as 1% for instance).
If the BSC determines that the FER is greater than the threshold, then the BSC will select a higher DGU, which will translate into a higher power level at which the BTS will transmit to the MS. On the other hand, if the BSC determines that the FER is less than the threshold, then the BSC will select a lower DGU, which will translate into a lower power level.